Recently, liquid crystal display device is used not only for monitors of personal computer but for televisions, and required therein is thinner display device with progressing large sized and high brightness display device. Therefore, further thinner members are also required for each member used for the liquid crystal display device as well as higher stability in quality.
Therefore, investigated is a polarizing plate in which cellulose triacetate film is provided a function of retardation with further thinning the film and enhancing stability thereof, because cellulose acetate film has excellent flatness and isotropy, and conventionally used to a protective film of a polarizing plate.
Since the cellulose triacetate film originally has high isotropy and weak birefringence, thereby it is not adapted to use for retardation film. Therefore, investigated are retardation films in which so called a retardation increasing agent is added into cellulose triacetate resin, or used is a mixed fatty acid ester of cellulose such as cellulose acetate propionate which has higher birefringence among cellulose ester resins (for example, Patent documents 1 and 2).
However, in order to have thinner retardation film, it was required to increase addition amount of a retardation increasing agent per thickness of the retardation film in which a retardation increasing agent is added. It sometimes causes problems such as bleed out which is a precipitation of additives from film.
Further, even though a retardation film using mixed fatty acid esters of cellulose has higher birefringence than cellulose triacetate, in order to have thinner film, required is further higher stretching ratio and it results in higher difficulty to produce film stably. Further, it is also difficult to synthesize mixed fatty acid ester of cellulose itself and there is further problem in low cost production of polarizing plate.
As a protective film for liquid crystal cell side of polarizing plate which is required to have function of optical compensation, inventors of the present invention investigate a protective film also having a function of a retardation film by realizing retardation by employing cellulose ester having acyl group total substitution degree of 2.1 to 2.6 which has high birefringence. As a result, a production of the film has been achieved by using material which has excellent retardation, and further enable to be thinner and low cost.
However, when a liquid crystal display device is prepared by using the obtained polarizing plate, a slight unevenness like stripe was observed in displaying black image. Further, when it was stored for long period in a high temperature and high humidity environment, unevenness like wave was observed in displaying image and it deteriorated visibility.
These problems have to be solved specifically when it is employed as a large size liquid crystal display device such as large size television or outer monitor and under severe conditions than conventional use such as with high-intensity backlight or under external environment